Salt bath



Patented Aug. 13, 1929.

UNITED ST .ARTHURE. BELLIS, BRANFOBD, CONNECTICUT.

SALT BATH.

No Drawing. Application filed September This invention relates to salt baths such as are used in the heat treatment of metals and for other purposes, and in general has for its primary objectand purpose to provide a salt bath composition containing in solution therewith a scavenging ingredient for the purpose of dissolving oxides and other impurities carried into the bath by the metalor entering the bath from the crucible wall, and thereby preventing contamination of the bath and maintaining the same at all times chemically neutral to the metal.

It is another object of my invention to provide an automatically acting scavenging ingredient for such salt baths which maybe advantageously used either in connection with a single phase bath of eutectic composition, or with a multi-phasc non-eutectic bath.

It is also the purpose of my present improvements to provide such a salt bath composition having maximum stability, high heat transfer efficiency and which will provide an ideal heating curve for the metal. In addition, my new bath is easily controllable as to its melting point within an appreciable temperature range, whereby its viscosity may be regulated or varied without effect upon its neutrality to the chemical composition of the metal being treated.

Specifically, my invention in its preferable embodiment comprehends the addition to ametalheat treating salt bath, such for instance as that disclosed in Patent No. 1,491,510, issued to The Bellis Heat Treating Company on April 22, 1924, of a boron compound, or the use alone of a suitable boron compound, such as fused borax. and boric acid, or the use of such compound with an alkaline chloride.

In the heat treatment of metals by immersion in a salt bath of either the eutectic single phase or the non-eutectic multi-phase type, I have succeeded in developing a bath composition which, even after excessive heating at high temperatures, maintained entirely free of. contamination by impurities such as metallic oxides. I have succeeded in accomplishing this result by using as one of the ingredients of. the bath, a soluble compound which subserves the purpose of a scavenger and immediately dissolves such metallic oxides or other impurities which may enter the bath. I have found that this result may best be obtained by the use of a borate, or boron trioxide.

10, 1925. Serial No. 55,623.

compound of boron, such as boric acid, a In practice, I have successfully used such boron compounds fused with an alkaline chloride which forms a slngle phase low melting point mixture. This mixture has a melting point of approximately 1330" F., and is very advantageous in the treatment of high grade tool steels at hardening temperatures from 1400 to 1 l50 F. This bath possesses the property of dissolving metallic oxides so that if such oxides are carried into the bath. by the metal being treated, as is usually the case, or if scale forms on the sides of the pot or crucible, these oxides are dissolved in the bath and do not decarburize the metal be- ,ing treated. Such oxide particles having a high melting point cannot, therefore, cling to the surfaces of the metal and affect the uniformity of itsheating or the rate of heat mg or react chemically with the metal, as by decarburizing steel. In other words, the bath is at all times maintained as a clear homogeneous solution without the accumulation therein of small solid particles of metallic oxides. It has been found that in order to obtain uniformly successful results in the heat treatment of metals in such a salt bath solution, the absence of metallic oxides is of great importance. Thus, first the efiiciency of the bath as a heating medium is greatly increased, secondly, the tendency of the bath to decompose or undergo changes in composition due to chemical reaction with such oxides is eliminated, thirdly, the viscosity of the bath may be controlled by regulation of its melting point temperature through increasing; or decreasing the proportion of the borates in the bath, thus enabling the metal to be treated through a desired temperature range, and fourthly, the protective action of the bath film adhering to the surfaces of the metal when it is withdrawn from the solution, is greatly improved.

Most tool steels are hardened at a temperature from 1400 to ll50 F., so that a liquid bath of relatively low viscosity at these temperatures is necessary for theproper hardening of the steel. I have found that in treating piecesof complicated shape having varying cross sections, there is less liability of the metal to warp if it is given a slow initial heating. Therefore, it will be evident that an ideal bath is one which, when the metal piece is immersed therein, will form a solid coating of the bath constituents around and upon the surface of the metal piece at temperatures below the transformation or hardening temperature of the steel. This coat of solid salt is immediately formed upon the immersion of the cold metal piece in the bath. However,.as the temperature of the metal increases, the coating is melted off and is entirely reconverted into liquid form at a temperature just below the hardening temperature of the metal. For the treatment of such metal pieces, I have developed a bath having a composition before fusion of 79% to 97% potassium chloride (KCl) and 3% to 20% anhydrous sodium metaborate. A bath of this composition has been found to be exceedingly stable, nonhygroscopic, neutral to the steel over a wide range of temperatures, and possessing the desired scavenging property above referred to whereby the bath solution is at all times maintained free from contaminating impurities. Various mixtures \of the above salts between the limits in their relative proportions as given may be produced for the treatment of different metals at, relatively different temperatures as may be required.

The mixture of the potassium chloride with the boron compound within approximately the proportions above stated results in an eutectic composition having a single liquid phase above its melting point. If, however, the proportion of either ingredient is increased or decreased relative to the other, then the composition is unbalanced and a second liquid phase will form in the composition. In certain cases, such a two phase liquid bath'may be found desirable, and in their case, due to the presence of a boron compound in the solution, the assurance of the entire freedom of the bath from contaminating impurities is obtained.

In the treatment of high speed tool steels, I have found it advantageous to use a mixture consisting of fused borax (Na,.B,O,)

I plus boric acid'fl-I BO or (Na O.B O plus (B 0 or such boron compounds with an alkaline chloride.

rax bead test of qualitative analysis). Therefore, a tetraborate, or any salt of H 80 whenadded to a eutectic salt bath will maintam the single phase character of the bath, or when added to a non-eutectic bath will malntain its-two phase character owing to the scavenging action as heretofore explained. I

llor hardening high speed steels, a bath wh ch is stable at temperatures up to 2500 F. is required so that the bath will notdeteriorate and also be neutral to the chemical composition of the steel. For such a bath, I have successfully used 31% boric acid,

49% anhydrous sodium metaborate and 20% of a potassium salt such as potassium nitrate which forms a complex acid borate of sodium and potassium. This I heating bath is used in connection with a quenching bath consistin of a eutectic mixture of potassium chloride and sodium carbonate and I have found it particularly applicable and desirable in the heat treatment of high carbon high speed steels.

From the above, it will be seen that my present improvements consist essentially in the use of Na BJ) or H 130, in eutectic or non-eutectic salt bath compositions, or the bath. At the same time, the high heat trans fer efliciency of the bath is maintained and it is possible by varying the quantity of the boron compound to change the melting point temperature and regulate the viscosity and also provide a protecting film on the surfaces of the metal piece of a desired thickness re- .quiring a predetermined period of time for the melting off or reconversion of such film.

into a liquid state after the cold metal is in serted' into the bath. Thus, the proper rate of heating of the piece and uniformity of the transfer of heatby the bath to all portions of the metal structure may be realized. The lack of such uniformity the heating of the metal is a serious deficiency of other metal heat treatment methods, as the nonuniform heating of the piece invariably results in the formation of dark spots or areas on the surface of the metal, which dark spots indicate improper heating and varying structure in the metal.

It will be understood from the foregoing that I do not 'necessarilyregard my present improvements as limited to the use of any Practically all oxides (as of Na, K, 1321,. Fe etc.) fuse-"with B 0 to give borates (boparticular boron compound in the bath solution, or to any particular relative proportions of the boron compound and the other bath constituents. Accordingly, the privilege is reserved of resorting to all such legitimate changes in'these respects as may be fairly considered within the spirit and scope of the invention as claimed. In the several examples above given the proportionate quantities of the salt components before fusion is stated, and it is possible that in the fusion of these components, certain complex salts may be formed and that, therefore, the fuse mixture, upon analysis would not be found contain said components in the ex act original proportions. It is, however, un-

necessary to consider such complex salts as may be formed in the bath solution, since I have found that the characteristics of the bath solution as above referred to are not 5 materially effected.

The word neutral as used in this specification has reference to chemical neutrality. In other words, the bath is of such composition that it does not in any way change the 10 chemical character of the metal being treated, as by carburizing or decarburizing the same or by any solvent action on the metal,

such as Would result from the use of an acid bath.

I claim:

1. A salt bath for heat treating metals consisting of substantially 79% potassium chloride and 20% boron compound.

2. A salt bath for heat treating metals consisting of substantially 7 9% potassium chloride and 20% anhydrous sodium borate.

In testimony that I claim the foregoing as my invention, I have signed my name hereto.

ARTHUR E. BELLIS. 

